The present invention relates to pattern inspection and, in particular, it concerns methods for processing high resolution images to identify defects on patterned surfaces, such as reticles, photomasks, semiconductor wafers, flat panel displays and other patterned objects.
It is known to generate optical, e-beam or other types of images of various patterned surfaces for the purpose of detecting defects. Image comparison is typically used in order to identify defects. Conventional techniques may be subdivided into two general classes, referred to herein as “die-to-database” and “die-to-die” comparisons.
In die-to-database comparisons, the image is compared to reference information typically corresponding to a CAD design according to which the pattern was produced. While this approach seems intuitively straightforward, practical implementations based upon this idea encounter numerous problems. Specifically, the database information typically corresponds to an idealized binary pattern where every point is either conductor or insulator, transparent or opaque, or the like. In contrast, the acquired image is a noisy multilevel grayscale image where intended features and defects are distinguished by subtleties of shading. In order to facilitate comparison, various theoretical models may be used to generate a synthesized grayscale reference image. Such models, however, are difficult to implement, and do not always provide reliable results.
In order to avoid these problems, many inspection systems employ die-to-die comparison wherein images of equivalent regions of adjacent dice on a wafer are compared directly. This is highly effective for identifying differences between dice, but does not indicate which die is defective. For this purpose, three-neighbor comparison is typically used to identify the defective die based upon a statistically justified assumption that a random defect will not occur at the same position in two dice.
One obvious limitation of die-to-die comparison is that it is only applicable where multiple copies of an identical pattern are available for comparison. In applications such as mask inspection where a single non-repetitive pattern is to be inspected, inspection systems generally revert to database-type comparison with its serious shortcomings as mentioned above.
An additional more subtle problem with die-to-die comparison limits the sensitivity of the technique, even where it is applicable. Specifically, the distance between the patterns being compared is necessarily the inter-die spacing. Both production techniques and imaging techniques generally introduce spatially-variable noise which is sufficient over the inter-die spacing to significantly degrade the sensitivity of the die-to-die comparison. Thus, for example, in the special case of a memory chip where the internally repetitive structure permits cell-to-cell comparison, a threshold of about 6 grayscale levels (out of 256) may be sufficient to reliably avoid false defect detection whereas, for die-to-die comparison necessary for logic chips, the threshold needed to be raised to 20 grayscale levels to avoid false alarms.
There is therefore a need for a pattern inspection technique which would facilitate image-to-image comparison within a single non-repetitive pattern.